Promoting Dual Electronic and Ionic Transport in PEDOT by

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Promoting Dual Electronic and Ionic Transport in PEDOT by Embedding Carbon Nanotubes for Large Thermoelectric Responses Kyungwho Choi, Suk Lae Kim, Suin Yi, Jui-Hung Hsu, and Choongho Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06850 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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ACS Applied Materials & Interfaces

Promoting Dual Electronic and Ionic Transport in PEDOT by Embedding Carbon Nanotubes for Large Thermoelectric Responses Kyungwho Choi,1,2+ Suk Lae Kim,1+ Su-in Yi,1 Jui-Hung Hsu,3 Choongho Yu1,3* 1

Department of Mechanical Engineering Texas A&M University College Station, Texas 77843 USA

2

New Transportation Systems Research Center Korea Railroad Research Institute Uiwang-si, Gyeonggi-do, 16105 Korea

3

Department of Materials Science and Engineering Texas A&M University College Station, Texas 77843 USA + *

Contributed equally to this work.

Corresponding author. Email: [email protected]

Keywords thermoelectric; poly(3,4-ethylenedioxythiophene); carbon nanotube; Soret effect; thermopower; ion transport

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Thermoelectric (TE) energy conversion with non-traditional organic materials is promising in wearable electronics and roll-to-roll manufacturing due to mechanical flexibility, lightweight, and easy processing. While typical organic materials have a benefit of low thermal conductivity that creates a large temperature gradient, relatively small thermopower (or Seebeck coefficient) often requires copious number of TE legs to fabricate practical TE devices. Here we show that hybrids of poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos) and carbon nanotubes (CNTs) can produce extremely large thermopower, ~14 mV/K at room temperature by a chemical reduction. With decent electrical conductivity, an extraordinary power factor of ~1200 µW/m-K2 at room temperature was observed. The large power factor could be attributed to prominent dual electronic and ionic conduction, which are likely to be promoted by embedding the CNTs in PEDOT due to the improved carrier mobility, in comparison to the inferior and scattered thermoelectric properties of PEDOT-only samples. While a higher CNT concentration gave a larger electronic contribution, a longer reduction or a lower CNT concentration provided a larger ionic contribution. Meanwhile well-separated CNTs created CNT junctions intervened by PEDOT-Tos, suppressing thermal transport. Further research utilizing the high thermoelectric responses could greatly help to develop practical wearable and/or mass-producible thermal energy harvesting and storage devices.


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INTRODUCTION TE energy harvesting from the environment or power consuming systems is very effective to build self-powered stand-alone devices, and TE cooling is the unique solution for systems requiring small form factors with robustness and silence. Current commercial TE devices are made of inorganic materials containing toxic, heavy, and/or rare-earth elements, posing problems in manufacturing, price, and mobile applications. The rigid structures of inorganic TE devices may not be suitable for systems with non-flat geometries such as wearable devices due to the poor thermal contact. Fully organic TE materials could potentially solve or alleviate the current problems due to their mechanical flexibility and simple manufacturing processes with less-toxic and earth abundant elements. In the past, organic materials were excluded as candidates for TE applications mainly due to their low electrical conductivity and small thermopower. Early reports in organic TE materials1-5 demonstrated the feasibility of increasing electrical conductivity while thermal conductivity and thermopower were not noticeably decreased. While various polymers3, 6-9

including highly conducting polyaniline10-15 were introduced, PEDOT1-2, 5, 16-19 had superior

performances mainly due to high electrical conductivity. A recent paper2 showed that thermopower can be raised using chemical reduction of PEDOT with tetrakis (dimethylamino) ethylene (TDAE). Nevertheless, this large increase in thermopower considerably decreased electrical conductivity due to the reduction of the electronic carrier concentration, resulting in a ZT lower than those of inorganic counter parts. CNTs have also been used as main ingredients20-26 or additives1,

3, 8, 15, 27-28

for TE

applications, but the relatively low thermopower of pristine CNTs is the roadblock to obtain a high TE performance. While the relatively small thermopower originating from pristine CNTs



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needs to be overcome to further improve the performance of the CNT-based TE devices, the high electronic carrier mobility29 up to 105 cm2/V-s could play a key role in raising electrical conductivity

without

significantly

affecting

thermopower

(i.e.,

decoupling

electrical

properties).30 As aforementioned, the common problem of conducting polymers and CNTs is the relatively low thermopower, requiring many serially connected n/p-type legs for an actual device operation.21 For instance, a TE material with 50 µV/K at a temperature difference of 10 K needs 2000 legs to attain 1 V. To boost the magnitude of thermoelectric voltage, it may be possible to utilize ionic transport in addition to the traditional electronic thermoelectric effect. For ionic conductors, it has been found that a large thermally induced voltage on the order of 1~10 mV/K can be generated.31-36 However, the electrical conductivity of typical ionic conductors is several orders of magnitude lower than those of traditional TE materials. Here we report an extremely large thermopower from CNT/PEDOT with a decent electrical conductivity by a TDAE reduction method for suppressing the electronic carrier transport and promoting the ionic transport, in comparison to the dominant electronic transport for highly conducting polymers. The reduction of hole carriers in PEDOT-Tos by the TDAE exposure decreased electrical conductivity, but the addition of CNTs to PEDOT-Tos more or less compensated the decreased electrical conductivity. The concentration of CNT was varied in order to alter the amount of PEDOT intervened between CNTs as well as the number of CNTs in direct contact. This study could provide insight about the role of CNT in electrical transport as well as the influence of CNT-PEDOT contacts on thermal conductivity. To have concrete ideas about the performance of our fully organic composites, we have measured their traditional thermoelectric performance indices, thermopower (S), electrical


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conductivity (σ) and thermal conductivity (k), and then calculated the power factor (S2σ) and figure-of-merit (ZT), which are considered most important in traditional thermoelectric materials. Our experimental results showed that an optimum combination of PEDOT-Tos, TDAE exposure, and CNT in our hybrid material have a relatively large ZT, which can be defined as S2σT/k where T indicate absolute temperature. The following describe the roles of CNT and TDAE treatment of PEDOT-Tos as well as electrical/thermal properties and material preparation/characterization.

RESULTS AND DISCUSSION Five types of CNT/PEDOT-Tos samples were fabricated by varying the concentration of CNT along with PEDOT-Tos only and CNT only samples. The concentration of CNT was varied by spraying CNT solutions on substrates for 5, 15, 30, 45, and 90 seconds to have very low (VL), low (L), medium (M), high (H), very high (VH) densities of CNTs, and representative scanning electron microscope (SEM) images are respectively depicted in Fig. 1a, b, c, d, and e (the hybrids with these CNT samples were labeled as Sample VL, L, M, H, VH). With 5-sec spraying, only a small fraction of CNT bundles were connected, as indicated by lines in Fig. 1a, and more CNT bundles was connected with longer spraying times. To have CNT/PEDOT-Tos 3,4-ethylenedioxythiophene (EDOT) was polymerized with CNTs. PEDOT only samples were also prepared by the same polymerization method without CNT for comparative study. Then, the hybrid films were exposed to TDAE vapor for 10, 30, and 60 min for reducing the hole carrier concentration by donating electrons to PEDOT.2,

35

In

general, traditional TE materials show increasing thermopower with a reduction of the majority electronic carrier concentration. In our samples such as Sample L, the low thermopower value (44 µV/K) of the samples before the TDAE exposure was remarkably increased up to ~14 mV/K



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with an exposure time of 30 min and then saturated at 60-min exposure, as depicted in Fig. 2. Similar behaviors were observed from Sample M. Our best power factor was measured to be ~1200 µW/m-K2 at 300 K (Fig. 2c). Measurement methods are described in detail in Section 1 of Supplementary Information. At the saturated reduction time (30 min), Sample VL containing less CNTs showed a lower electrical conductivity with a similar thermopower, resulting in a lower power factor. On the contrary, Samples H and VH where many CNTs formed percolated networks had higher electrical conductivities with lower thermopower values. The % reduction of the thermopower was comparable to the % increase of the electrical conductivity, but the power factor became lower because of the square term (S2). Our thermopower value, 14 mV/K is extremely large, compared to the previously reported thermopower of typical conducting polymers and organic composites (> GIonic, S ≈ SElectronic, which is the lower right part of the plot. The upper left corner on the plot is the other extreme case. 8 ACS Paragon Plus Environment

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Here we noticed the major difference between Fig. 3c and d (with and without CNT) is the data points appeared on the lower left corner for the PEDOT-only samples while the data for CNT/PEDOT are close to the linear line on the log-log plot, meaning that the data closely follow S~σ

−a

relation, where a = 0.598 was used to fit the data. A similar power law, S ~ σ

−1/4

was

also observed from the thermoelectric properties of other polythiophenes “without” CNT.53 These thiophenes have more crystalline structures compared to PEDOT and/or do not contain the PSS copolymer or Tos. Coincidently, the CNT/PEDOT composites have more crystalline features due to the CNT and π-π interactions between CNT and PEDOT,35 compared to PEDOT by itself. In general, as the crystallinity of polymeric materials gets higher, the mobility is raised and thereby electrical conductivity is improved. In fact, it was found that the ionic and electronic electrical conductivity can be enhanced by adding CNTs to polymers.5, 14, 54 It appears that the CNT addition helps to promote the carrier transport, which pushes the properties toward the trendline on the plot by improving electrical conductivity or/and thermopower. Nevertheless we cannot exclude the possibility of simple variations arising from different processing methods and copolymers as well as different humidity conditions such as the extreme case of both the low thermopower and electrical conductivity on the lower left corner for an ionic conductor at a low humidity (deactivated ionic carriers and lack of electronic carriers). The thermal conductivity of Sample L after TDAE treatment was measured using microfabricated devices55 with two suspended membranes (Fig. 4a) (see Section 3 in Supplementary Information for details). Note that both electrical and thermal properties were measured along the in-plane direction. When CNTs were added to PEDOT-Tos, the thermal conductivity was increased to ~0.52 W/m-K presumably due to the high thermal conductivity of CNTs. It should be noted that the electronic contribution based on the Wiedemann-Franz law is



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estimated to be much smaller than the total thermal conductivity. Despite the high thermal conductivity of an “individual” CNT, ~103 W/m-K,56 thermal transport through the interfaces between CNTs can be considerably deterred due to the thermal contact resistance at the polymer between CNTs and the point (or line) contact between CNTs.5, 57-58 The thermal conductivity of polymer composites are typically low, 0.2~0.4 W/m-K at 300 K with 20 wt% or less CNT concentrations,1, 3 unless the electronic contribution to thermal conductivity is high.59-60 In our samples, the sparsely distributed CNTs could impede thermal transport at the junctions between CNTs so as to maintain relatively low thermal conductivity. The thermal conductivity of Sample L was also calculated using the Monte Carlo method61 (Fig. 4c) (see Section 4 in Supplementary Information for details). Here average diameter (20 nm) and length (1.5 µm) of CNT bundles were obtained from the SEM images of Sample L, and then the straight CNTs of the same diameter and length with the same CNT volume fraction were generated at random locations with random orientations (inset of Fig. 4c). The thermal conductivity of the hybrid was calculated as a function of the thermal conductivity of CNTs. With the thermal conductivity of the hybrid (0.52 W/m-K), the corresponding thermal conductivity of the CNT portion was found to be 60 W/m-K, which is close to that of the unaligned CNT mat.62 Figure 4d shows ZT of Sample L along with Sample VL and M at different reduction times. The maximum ZT at 300 K from Sample L was found to be ~0.7. Note that the meaning of ZT here may be different from that of traditional thermoelectric materials because our samples have both ions and electrons as transport carriers. It is worth noting that the thermal conductivity of Sample L was not strongly affected by the reduction time due to the small electronic contribution. The thermal conductivity values of Sample L before and after the reduction were


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similar (see Fig. S5 for the measurement results of Sample L before the reduction). The ZT values of Sample VL and M were also calculated using their thermal conductivities obtained from the Monte Carlo calculations with the thermal conductivity of CNTs (60 W/m-K) as an input parameter.

CONCLUSION Fully organic composites consisting of PEDOT-Tos and CNT were synthesized and the composites were treated with TDAE to maximize thermopower. After 30-min TDAE treatment, thermopower became extremely large, ~14 mV/K while electrical conductivity was maintained to a decent level due to the inclusion of CNT, resulting in a large power factor of ~1200 µW/mK2 and ZT of ~0.7 at room temperature. While the large increase in the thermopower of PEDOTTos suggests the presence of ionic transport, it appears that the CNT addition promotes both ionic and electronic transport, displaying better thermoelectric properties compared to PEDOTonly samples. The thermal conductivity of our composites was measured to be low despite the presence of CNT as a result of deterred thermal transport at the CNT junctions. We anticipate that this p-type PEDOT-Tos with TDAE could be an excellent pair with our earlier n-type PEDOT-FeCl4− work, which may serve as a foundation to develop thermal energy harvesting devices producing output voltages that are high enough to be utilized in practice.

MATERIALS AND METHODS Material synthesis. A CNT solution was prepared by dispersing 2-mg of singe-wall CNTs (P2 grade, Carbon Solutions, Inc.) in 20-mL of deionized (DI) water with 6-mg of sodium dodecyl benzene sulfonate (SDBS) (88%, Acros organics) with a bath type sonicator (Branson 1510) for



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2 hours and then a probe sonicator (48 W, Fisher Scientific FB120) for 2 hours. This process was repeated three times, and then the solution was centrifuged at 12,000 rpm for 20 minutes (Fisher Scientific accuSpin Micro17). The upper ~70% of the supernatant solution was carefully decanted and directly sprayed with a spray gun (0.2 mm nozzle diameter, GP-S1, Fuso Seiki Co.) onto glass substrates at ~80 °C for varying time periods. Subsequently, the samples were immersed in DI water for 10 minutes to remove SDBS and then the water was blow-dried by air in ambient conditions. The images in Fig. 1 were taken after this process. To synthesize PEDOTTos, a monomer solution was prepared by adding 126-mg of EDOT (98+%, TCI) to an oxidative solution containing 2.03-g of iron (III) tris-p-toluenesulphonate in n-butanol (38-42 wt%, Clevios C-B 40 V2), 2.03-g of n-butanol (99.4%, EMD), and 56-mg of pyridine (99+%, Alfa Aesar). 0.24-mL of this solution was spin-coated on the CNT-sprayed glass substrates at 2000 rpm for 30 seconds. Subsequently, the samples were placed in a convection oven at 110 °C for 10 minutes for polymerization, and then naturally cooled down to room temperature (~30 minutes). Finally, the samples were immersed in DI water to remove excessive iron tosylate for 10 minutes and blow-dried by air. The film thickness was measured to be 80-110 nm using a surface profilometer (KLA-Tencor P-6). For the TDAE treatment, vapor from a few drops of TDAE (85%, Sigma Aldrich) were exposed to the sample. The reduction process was performed at room temperature in a 68-70 kPa vacuum environment with 30% relative humidity. The reduction level was controlled by varying the TDAE exposure time. The PEDOT-Tos only sample was prepared by the same method described above except a glass substrate instead of the CNT-coated substrates. The CNT only sample was prepared by spraying the CNT supernatant solution on a glass substrate at ~80 °C with the spray gun for 200 sec. The sample thickness was measured to be 40 nm.


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Electrical property measurements. In-plane electrical conductivity was measured using a four probe method at room temperature . For in-plane thermopower measurements, TE voltage at 6-8 different temperature gradients between -10 and +10 K was used at room temperature (see Section 1 in Supplementary Information). The electrical property measurement techniques were verified with control experiments with single-crystalline silicon.35

Acknowledgements The authors gratefully acknowledge financial support from the U.S. Air Force Office of Scientific Research (Grant No. FA9550-13-1-0085) and the R&D Program of the Korea Railroad Research Institute.

Supporting Information Brief statement in electrical and thermal property measurement methods and supplementary data. Detail descriptions about the Monte Carlo simulation methods and related data.



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Table 1. Thermoelectric properties of the relevant CNT/PEDOT composites at room temperature in this work and literature. The measurement conditions were assumed to be ambient unless noted otherwise in literature. Only two significant digits were taken for the data comparison. Materials

Measurement condition

σ (S/m)

S (µV/K)

Reference

3.2 1.3×104 6.4 1.4×104 8.5 9.9×103 81 2.5×103 2.5×102 4.6×102 This work Ambient 3 36 9.0×10 44 9.2×103 37 9.8×103 43 1.2×104 4 37 2.3×10 5 59 1.3×10 38 1.8×105 Ambient 30 59 4.0×102 3 50 1.6×10 2 Dry −1.2×103 6.5×10 PEDOT/FeCl4/SWCNT+TDAE 35 Ambient 6 −7.6×103 PEDOT:PSS(PH500)/SWCNT(60%) 29 1.3×105 Ambient 5 PEDOT:PSS(PH1000)/SWCNT(60%) 35 9.5×104 PEDOT:PSS/SWCNT, layered 39 8.0×103 Ambient 41 4 PEDOT:PSS/SWCNT+DMSO, layered 30 2.4×10 PEDOT-block-PEG/SWCNT(20%) 37 3.2×103 PEDOT-block-PEG/SWCNT(33.3%) 53 Ambient 42 4.5×103 PEDOT-block-PEG/SWCNT(66.7%) 48 8.5×103 PEDOT-Hexane/SWCNT 18 5.5×104 Ambient 43 PEDOT-Xylene/SWCNT 16 3.0×104 PEDOT:PSS/SWCNT(20%) 26 6.0×104 Ambient 44 18 PEDOT:PSS/SWCNT(60%) 1.9×105 PEDOT:PSS/SWCNT(95%) 16 3.6×105 PEDOT:PSS/DWCNT(20%)+EG 44 7.8×104 Ambient 45 PEDOT:PSS/DWCNT(25%)+EG 45 6.5×104 4 PEDOT:PSS/DWCNT(30%)+EG 35 7.4×10 Acronyms: SWCNT: Single-wall CNT; DWCNT: Double-wall CNT; PEG: Polyethylene glycol; DMSO: Dimethyl sulfoxide; EG: Ethylene glycol PEDOT-Tos/SWCNT(VL)+TDAE PEDOT-Tos/SWCNT(L)+TDAE PEDOT-Tos/SWCNT(M)+TDAE PEDOT-Tos/SWCNT(H)+TDAE PEDOT-Tos/SWCNT(VH)+TDAE PEDOT-Tos/SWCNT(VL) PEDOT-Tos/SWCNT(L) PEDOT-Tos/SWCNT(M) PEDOT-Tos/SWCNT(H) PEDOT-Tos/SWCNT(VH) PEDOT:PSS/SWCNT(6.7%)+DMSO PEDOT:PSS/SWCNT(20%)+DMSO PEDOT:PSS/SWCNT(6.7%) PEDOT:PSS/SWCNT(20%)


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Table 2. Thermoelectric properties of the relevant PEDOT samples at room temperature in this work and literature. The measurement conditions were assumed to be ambient unless noted otherwise in literature. Only two significant digits were taken for the data comparison. Materials

Measurement condition RH 60% Dry Ambient

σ (S/m)

S (µV/K)

Reference

PEDOT-Tos+TDAE 2.7 1.4×104 PEDOT-Tos+TDAE 1.1 This work 7.0×102 3 PEDOT-Tos 36 9.0×10 PEDOT/FeCl4+TDAE 34 −4.3×103 35 Ambient PEDOT/FeCl4 50 72 PEDOT-Tos+TDAE (oxidation 15%) 0.3 7.0×102 3 PEDOT-Tos+TDAE (oxidation 22%) Ambient 2 7.0×10 2.2×102 4 PEDOT-Tos+TDAE (oxidation 33%) 50 3.0×10 PEDOT:PSS (gate voltage 0V) 9 2.7×104 90 PEDOT:PSS (gate voltage 0.8V) Ambient 46 2.5×103 2 PEDOT:PSS (gate voltage 1.4V) 30 4.0×10 PEDOT-Tos 11 Ambient/Dry 1.5×104 PEDOT-S Ambient/Dry 80 8 PEDOT:PSS-DEG Ambient/Dry 6 5×102 PEDOT:PSS Dry 14 9 47 PEDOT:PSS Ambient 15 1.6×102 PEDOT:PSS-PSS:Na Dry 10 8.0×10-2 PEDOT:PSS-PSS:Na Ambient 4 2.1×102 PEDOT:Tos (potential V= -2.0) 4.5×103 1.9×102 4 Ambient 19 PEDOT:Tos (potential V= 0.0) 96 1.4×10 PEDOT:Tos (potential V= +2.0) 49 7.0×103 PEDOT:PSS-DMSO+EG 0 34 6.1×104 PEDOT:PSS-DMSO+EG 100 Ambient 73 48 8.9×104 4 PEDOT:PSS-DMSO+EG 200 70 9.7×10 PEDOT:ClO4 (reduction by hydrazine 11s) 33 3.5×104 PEDOT:PF6 (reduction by hydrazine 11s) 32 Ambient 49 5.7×104 PEDOT:BTFMSI (reduction by hydrazine 5s) 38 1.1×105 PEDOT:PSS 31 36 PEDOT:PSS+DMSO 41 Ambient 30 9.8×104 PEDOT:PSS+FA 39 1.1×105 PEDOT:PSS+DMSO RH 30% 17 8.1×104 50 PEDOT:PSS+DMSO RH 60% 49 8.7×104 PEDOT:PSS+hydrazine (10×10-5 M)+DMSO 49 1.3×105 -5 Ambient 51 PEDOT:PSS+hydrazine (15×10 M)+DMSO 50 1.2×105 PEDOT:PSS+hydrazine (20×10-5 M)+DMSO 48 1.0×105 Acronyms: DEG: diethylene glycol; BTFMSI: bis(trifluoromethylsulfonyl)imide; FA: formic acid



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Figure 1. CNTs sprayed on glass substrates for different time periods. (a) Very low density CNTs (slender objects in the image) with 5-sec spraying (Sample VL). (b) Low density CNTs sprayed for 15 sec (Sample L). (c) Medium density CNTs sprayed for 30 sec (Sample M). (d) High density CNTs sprayed for 45 sec (Sample H). (e) Very high density CNTs sprayed for 90 sec (Sample VH). All scale bars indicate 4 µm.


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Figure 2. Electrical properties of the hybrids vs. TDAE exposure time. Electrical conductivity (a), thermopower (b), and TE power factor (c) of Sample L and M after TDAE reduction for 10, 30, and 60 min; and those of Sample VL, H, and VH after 30-min reduction. The reduction effect was saturated after exposing the samples to the TDAE vapor for 30 min.



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Figure 3. Electrical properties of PEDOT-Tos, CNT/PEDOT, and PEDOT only samples. The electrical conductivity (a) and thermopower (b) of PEDOT-Tos before the TDAE treatment and after the TDAE treatment in a dry (Ar) and humid (RH 60%) environment. Thermopower vs. electrical conductivity for CNT/PEDOT (composite) (c) and PEDOT (d) in this work and literature. Note that the absolute values for n-type thermopower were plotted.


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Figure 4. Measured and calculated thermal conductivity of Sample L and PEDOT-Tos, and ZT of Sample VL, L, and M. (a) Sample L was mounted between two suspended membranes of a microfabricated device for thermal conductivity measurements. The scale bar indicates 20 µm. (b) Thermal conductivity (khybrid) of Sample L at 285~310 K. (c) Monte Carlo simulation of the thermal conductivity of Sample L vs. thermal conductivity (kCNT) of CNTs. The inset shows randomly distributed CNTs generated by the Monte Carlo simulation (a portion of 5×5 µm2). (d) ZT of Sample VL, L, and M at 300 K vs. TDAE exposure time. Here ZT was presented as a convenient comparison index but the meaning of ZT may be different from that of traditional inorganic thermoelectric materials.



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TOC Extremely large thermopower and decent electrical conductivity from polymer composites of carbon nanotubes and poly(3,4ethylenedioxythiophene) due to thermally driven ionic and electronic transport

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Promoting Dual Electronic and Ionic Transport in PEDOT by

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